Merge ExtractImagesNode into ImageSaver

[mikeferguson]

Functionality is largely the same, we should:

• Replace "secs_per_frame" by adding new "fps" parameter to ImageSaver - when the parameter is set, we should setup a timer to fire and save images. (see also sec_per_frame_ is unintuitive #274)
• Update file format to be "frameX" rather than "leftX"
• Use the encoding of the image message, falling back to the "encoding" parameter only when empty.